Fulmination of returning aid to ethnic group (8)
I believe the answer is:
diatribe
'fulmination' is the definition.
('diatribe' can be a synonym of 'fulmination')
'returning aid to ethnic group' is the wordplay.
'returning' shows that the letters should be reversed in order.
'to ethnic group' becomes 'tribe' (I've seen this before).
'aid' back-to-front is 'dia'.
'dia'+'tribe'='DIATRIBE'
'of' is the link.
(Other definitions for diatribe that I've seen before include "Loud verbal attack" , "Bitter words" , "Philippic" , "stream of abuse" , "slating" .)